1. Field of the Invention
This invention relates to improved analog to digital and digital to analog conversion techniques and more particularly to improved analog to digital and digital to analog conversion techniques using delta modulation.
2. Description of the Prior Art
Conversion of analog signals to digital data inherently involves different trade-offs. Most of these tradeoffs involve speed, quantization noise, dynamic range and cost.
For example, analog to digital conversion can involve either a serial process or a parallel process. Serial conversion frequently allows for a simpler and a less costly implementation but tends to be slower than parallel conversion.
A further consideration is the amount of noise that is permitted. The more bits that are used for quantifying the analog signals, the less quantization noise that will result from the conversion and the greater the dynamic range of the system. For example, using simple pulse code modulation (PCM), 128 separate levels as represented by 7 bits are required for a dynamic of 42 dB while 131,072 separate levels or 17 bits is required for a dynamic range greater than 100 dB.
Another way of expressing the noise problem is based upon the resolution of the digital to analog converter. A seven bit digital representation of a five volt input signal provides a resolution of 0.0391 volts (five volts divided by 128) ; i.e., the digital representation of the analog signal can only reflect changes greater than 0.0391 volts in the analog signal. However a 17 bit representation of the same signal can reflect changes greater than 3.81.times.10.sup.-5 volts. Of course, the original analog input signal, theoretically has an infinite resolution.
Another problem common with many analog to digital and digital to analog converters is dithering. The analog to digital converter's switching circuitry introduces objectionable noise into the converted signal at low signal levels or with slowly changing signals. To avoid such noise, complicated dithering circuits such as those shown in U.S. Pat. No. 4,490,714 to van de Plassche have been used.
One digital to analog system that may have relatively high dynamic range and low quantization noise is a delta modulation system, which is a one bit output form of a PCM system. FIG. 1 represents the classic delta modulation system 10. An input analog signal e.sub.i signal is fed through a comparator 12 with the output signal .epsilon. of the comparator 12 representing which of the input signal e.sub.i and an output e.sub.l of an integrator 14 is greater. That output .epsilon. is modulated by a modulator 16 supplied with a periodic pulse train from a pulse generator 18. In effect, the modulator 16 samples the output .epsilon. of the comparator 12. The output of the modulator e.sub.o is fed back for integration by an integrator 14.
The result is that the output e.sub.o of the delta modulator 10 is a series of pulses. The pulses represent the rate of change of the signal amplitude from sampling instant to sampling instant.
A system 20 to reconstruct the original input signal e.sub.i is shown in FIG. 2. The output e.sub.o of the delta modulator is integrated by an integrator 22 and is then inputted to a low pass filter 24 to eliminate high frequency noise caused by the sampling or the modulation. Alternatively, a frequency limited integrator may be used.
To reduce noise in the system, a higher frequency for the pulse train can be used. Further, instead of using just one integrator in the modulator and demodulator, two integrators may be used. Further details regarding different types of delta modulators may be found in a variety of textbooks, including Philip F. Panther, Modulation, Noise and Soectral Analysis (1965).
However, delta modulators have several disadvantages. They are relatively slow in responding to rapid, large changes in the amplitude of the input signal. Further, delta modulators lose all D.C. information contained in the original analog signal. In addition, delta modulators have limited resolution.
Delta modulators also cause quantization noise in the resultant analog output signal upon conversion back to an analog signal. Commonly, such quantization noise can be avoided by dithering.
Therefore, it is a first object of this invention to provide a relatively simple analog to digital converter that provides high speed conversion with good resolution. It is a further object of this invention to avoid the use of dithering to remove quantization noise and to avoid the loss of DC signals. And it is a still further object of this invention to provide such a converter that is simple and that may be incorporated into one integrated circuit.